Sent is the past tense send. Put differently, 'send' describes something you still need to do while 'sent' is something already done.For example: -I'll send you those e-mails later. -I've sent the e-mails already. Hope it helped.

I send a message to someone who sent me a message. 'I send' is present tense' and 'who sent' is past tense. The verb send is an irregular verb [send-sent-sent]. The infinitive is 'send', the past tense is 'sent' and the past participle is 'sent'.